Determination of the source area of the Ya13-1 gas pool in the Qiongdongnan Basin, South China Sea

The Ya13-1 gas field is the largest and earliest commercial gas field discovered in the Qiongdongnan Basin, South China Sea. Recent work has indicated the gas source to be a series of black mudstones in the Yacheng Formation of Eocene-Oligocene age in the Yanan Depression, Qiongdongnan Basin. However, there are no data indicating the effective source area. In this paper, methane generation kinetics and carbon isotopic fractionation kinetics were combined with fluid inclusion data to investigate the effective source area and gas migration and accumulation efficiency. Fluid inclusions from the gas reservoir rocks homogenize in the range of 118-173 °C. By combining the fluid inclusion data with the thermal history of the reservoir, the filling of the gas pool is deduced to have occurred from 2 Mabp to present. The results from geological models of gas generation and methane carbon isotopic fractionation of the source rock vary widely from the depression center to the position of the structure. The source rock has a methane fractional conversion ranging from 0.10 to 1.0, and the generated gas has predicted δ13C1 values in the range from −26.0‰ to −42.0‰. Compared with the δ13C1 values of gas from the gas pool, these models indicate that an effective gas source rock area is located in the slope area of the Yanan Depression, with a present burial depth of 4000-5500 m. On this basis, a model showing the gas generation, migration and accumulation for the Ya13-1 gas pool is proposed. It is believed that the methods and the models developed in this paper are important guides to further research and exploration for natural gases in the Yinggehai-Qiongdongnan Basin.